This invention relates to a pivot system for a microtome and, in particular, to a supplementary misalignment pivot arrangement for accommodating misalignment of elements in the main pivot system.
A microtome is a device adapted to provide extremely thin slices of a material for microscopic examination. Typically, the material to be sliced is mounted to the forward end of an articulable specimen holding arm which is itself pivotally mounted at its opposite end within a yoke. The yoke is itself pivotally mounted to the microtome frame and connected to an advance member, as a lead screw. In operation the specimen is drawn downwardly over the edge of a knife thereby slicing an extremely thin slice of specimen. In most instances, the slice is collected on the surface of a liquid reservoir. As the yoke is advanced the holding arm articulates thus permitting the specimen to be raised above the cutting edge of the knife in anticipation of the next downward cutting stroke.
In order to consistently and repeatably provide slices of the thicknesses required for the purpose under discussion extremely precise pivoting motion between all relatively movable members is an absolute requirement. Ball pivot assemblies are usually utilized to define the bearing surfaces for the pivotal movements above discussed. However, when ball pivots are used, there is a tendency for misalignments between the bearing balls and their associated sockets. Accordingly, when balls are used to define the surfaces on which the pivoting movement between the members occurs, some arrangement must be provided whereby the exact distances between the pivot balls and their corresponding sockets are maintained.
One method known in the art of achieving such exact matching distances between the pivot balls and their sockets is shown in Austrian Pat. No. 238,963. In this patent both of the pivot balls are fixed to one of two relatively pivotal members. However, each of these balls is supported in the other of the relatively pivotably members in a different manner. One of the balls is supported in a socket secured in a fixed location to the second member. The second of the balls is received in a substantially V-shaped groove provided in the second member. The corresponding ball may occupy any position within the groove thereby assuring exact matching of the distance between the balls and the distance between their corresponding support surfaces.
U.S. Pat. No. 4,221,438 (Sitte et al.) discloses an alternative technique. In one embodiment of the device shown in this patent one of the balls is introduced into a socket containing molten cement and permitted to situate itself such that when the cement sets up there is an exact matching of the distance between the balls and the sockets. In a second embodiment of the device shown in this patent one of the sockets is fixed to one of the members while the other of the bearing sockets is mounted on a leaf spring for flexural movement with respect to that member. In this manner deviations in the distances between the balls and the sockets may be accommodated so that exact matching of the bearing elements may be facilitated. However, the flexing of the leaf spring to accommodate misalignments between the sockets and balls results in the generation of a restoring force which is imposed on the system. This restoring force, acting in a direction parallel to the axis of the balls, may disrupt the precise slicing of sections of the required thickness.
Accordingly, it is believed advantageous to provide a misalignment system to support relatively pivotal members within a microtome so that perceived disadvantages attendant upon the use of flexural members does not occur.